Beta-Type Titanium Alloy

ABSTRACT

Disclosed is a β-titanium alloy consisting of, in weight percent, 5-15% of V, 0.5-2.5% of Fe, 0.5-6% of Mo, 0.5-5% of Cr, 1.5-5% of Al, and the balance of Ti and impurities. When the weight % of V content is expressed as X V , the weight % of Fe content is expressed as X Fe , the weight % of Mo content is expressed as X Mo , and the weight % of Cr content is expressed as X Cr ; the value of X V +2.95X Fe +1.5X Mo +1.65 X Cr  is 15-23%. Such a β-titanium alloy has excellent cold workability, while having higher strength than Ti-20V-4Al-1Sn β-titanium alloy.

FIELD OF THE INVENTION

The present invention relates to a β-type titanium alloy and a method for heat treatment thereof.

BACKGROUND OF THE INVENTION

Titanium alloys are light in weight and high in strength, and of them, there exist titanium alloys called as β-type titanium alloys that comprise only the β-phase, many of which are excellent in cold workability compared with titanium alloys that comprise mainly the α-phase, and many being able to be increased in strength by an aging treatment.

Examples of the known β-type titanium alloys include Ti-20V-4Al-1Sn (Patent Reference 1), Ti-15V-3Cr-3Al-3Sn, Ti-22V-4Al (Patent Reference 2), Ti-15V-6Cr-4Al (Patent Reference 3), Ti-13V-9Cr-3Al, Ti-15Mo-5Zr-3Al, Ti-3Al-8V-6Cr-4Mo-4Zr, Ti-13V-11Cr-3Al, and Ti-4.5Fe-6.8Mo-1.5Al.

Of them, Ti-15V-6Cr-4Al, Ti-13V-9Cr-3Al, Ti-15Mo-5Zr-3Al, Ti-3Al-8V-6Cr-4Mo-4Zr and Ti-13V-11Cr-3Al are high in strength, but have a high deformation resistance in cold working and hot working, deteriorating the workability thereof, and thus are limited in application to special fields. On the other hand, Ti-20V-4Al-1Sn, Ti-15V-3Cr-3Al-3Sn and Ti-22V-4Al are slightly lower in strength but are excellent in cold workability and thus are broadly and generally used. Of them, Ti-20V-4Al-1Sn is used in sports products, such as golf clubs and bicycles, as well as various other products, because of its excellent cold workability and relatively high strength.

In recent years, β-type titanium alloys are required to be increased in strength for further application in various fields, further weight reduction and cost reduction, and specifically required to have a higher strength while having excellent workability as the Ti-20V-4Al-1Sn does. However, no β-type titanium alloys that have excellent cold workability and a higher strength than Ti-20V-4Al-1Sn have been found through the studies made up to the present, and therefore the above demands have not yet been satisfied.

Patent Reference 1: Japanese Patent No. 2640415

Patent Reference 2: Japanese Examined Patent Application Publication No. Hei-6-99765

Patent Reference 3: Japanese Unexamined Patent Application Publication No. 2000-144286

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In consideration of the above problems, it is an object of the present invention to provide a titanium alloy that has excellent cold workability, and higher strength than Ti-20V-4Al-1Sn β-type titanium alloy.

Means to Solve the Problems

The present inventors made intensive studies in order to solve the above problems, and found that there can be provided an indicator for precisely indicating the degree of stabilization of the β-phase, by determining the content of each of β-phase stabilizing elements, namely V, Fe, Mo and Cr, not on the basis of a general method using the ratio of the minimum amount to be added for β-phase stabilization when they are added independently of each other to titanium, but by the use of a novel coefficient conceived in consideration of the interaction between the respective elements.

More specifically, the respective elements contained in a β-type titanium alloy are given indices on β-phase stabilizing effects realized by them, which indices each are generated by a reciprocal number of the minimum amount enabling a titanium alloy to comprise only the β-phase by each element, and as a general knowledge, it is known that a titanium alloy can comprise only the β-phase by containing, by mass %, V: 15%, Fe: 3.6%, Mo: 10% and Cr: 6.3%, and therefore when designating V as a reference, the numerical value determined by multiplying the mass % of the Fe by 15/3.6 is considered as an equivalence when the said numerical value of V is contained.

Accordingly, it is assumed from the conventional way of thinking that the degree of stabilization of the β-phase can be determined by the value of X_(V)+(15/3.6)X_(Fe)+(15/10)X_(Mo)+(15/6.3)X_(Cr) when V is designated as a reference, in which X_(V) represents the mass % of the V contained, X_(Fe) represents the mass % of the Fe contained, X_(Mo) represents the mass % of the Mo contained, and X_(Cr) represents the mass % of the Cr contained. However, from the studies based on experiments made by the inventors et al., it was found that the employment of the value of X_(V)+2.95X_(Fe)+1.5X_(Mo)+1.65X_(Cr) can provide an index for precisely indicating the degree of stabilization of the β-phase.

That is, according to the present invention, there is provided a β-type titanium alloy characterized by comprising, by mass %, V: 5 to 15%, Fe: 0.5 to 2.5%, Mo: 0.5 to 6% and Cr: 0.5 to 5%, wherein the value of X_(V)+2.95X_(Fe)+1.5X_(Mo)+1.65X_(Cr) is from 15 to 23%, wherein X_(V) represents the mass % of the V, X_(Fe) represents the mass % of the Fe, X_(Mo) represents the mass % of the Mo and X_(Cr) represents the mass % of the Cr, and further comprising, by mass %, Al: 1.5 to 5%, wherein Ti and impurities constitute the residue.

ADVANTAGES OF THE INVENTION

According to the present invention, it is possible to provide a titanium alloy that is more excellent in strength than the Ti-20V-4Al-1Sn β-type titanium alloy due to the solid solution hardening action, while having excellent cold workability as compared with a Ti-20V-4Al-1Snβ-type titanium alloy, by containing Fe, Mo and Cr in addition to the V, and specifically containing, by mass %, V: 5 to 15%, Fe: 0.5 to 2.5%, Mo: 0.5 to 6%, and Cr: 0.5 to 5%, wherein the value of X_(V)+2.95X_(Fe)+1.5X_(Mo)+1.65X_(Cr) is from 15 to 23%, in which X_(V) represents the mass % of the V, X_(Fe) represents the mass % of the Fe, X_(Mo) represents the mass % of the Mo and X_(Cr) represents the mass % of the Cr.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Now, the description will be made for the reason for determining the content of each element in a titanium alloy of this embodiment.

The titanium alloy of this embodiment contains, by mass %, V: 5 to 15%, Fe: 0.5 to 2.5%, Cr: 0.5 to 5% and Al: 1.5 to 5%, and Ti and impurities, in which the Ti and the impurities constitute the residue.

It is possible to provide a β-type titanium alloy, which has excellent cold workability, by melting a titanium alloy of these elements and rapidly cooling the same.

Then, the β-type titanium alloy is processed into a desired shape and then is subjected to a heat treatment which is called as an aging treatment to have the α-phase, which has a higher strength than the β-phase, precipitated in the β-type titanium alloy and thereby can be increased in strength.

V is contained, by mass %, within a range from 5 to 15% because when the content of V is less than 5%, the cold workability of a β-type titanium alloy is deteriorated and thus excellent cold workability is not obtainable; and when the content of V exceeds 15%, the precipitation of the α-phase in the aging treatment is inhibited and hence more excellent strength than Ti-20V-4Al-1Sn is not obtainable.

Fe is contained, by mass %, within a range from 0.5 to 2.5% because when the content of Fe is less than 0.5%, an advantage of solid solution hardening action is not obtainable and hence more excellent strength than Ti-20V-4Al-1Sn is not obtainable; and when the content of Fe exceeds 2.5%, segregation of Fe occurs in a β-type titanium alloy and hence unevenness in characteristics occurs.

Mo is contained, by mass %, within a range from 0.5 to 6% because when the content of Mo is less than 0.5%, an advantage of solid solution hardening action is not obtainable and hence more excellent strength than Ti-20V-4Al-1Sn is not obtainable; and when the content of Mo exceeds 6%, excellent cold workability is not obtainable. Furthermore, Mo is an expensive material and therefore a problem of increasing costs is caused by increasing the content thereof.

Cr is contained, by mass %, within a range from 0.5 to 5% because when the content of Cr is less than 0.5%, an advantage of solid solution hardening action is not obtainable, and hence more excellent strength than Ti-20V-4Al-1Sn is not obtainable; and when the content of Cr exceeds 5%, segregation of Cr occurs in a β-type titanium alloy and hence unevenness in characteristics occurs.

Al acts on the stabilization of the α-phase while V, Fe, Mo and Cr are elements for stabilizing the β-phase, and Al is contained, by mass %, within a range from 1.5 to 5% because when the content of Al is less than 1.5%, the precipitation of the α-phase by an aging treatment cannot be accelerated, and hence more excellent strength than Ti-20V-4Al-1Sn is not obtainable. Furthermore, Al has an advantage of inhibiting the precipitation of the ω-phase, and therefore the ω-phase may be precipitated and thereby the alloy may be weakened, when the content of Al is less than 1.5%.

When the content of Al exceeds 5%, the cold workability is deteriorated and hence excellent cold workability is not obtainable.

The contents of V, Fe, Mo and Cr are set so that the value represented by X_(V)+2.95X_(Fe)+1.5X_(Mo)+1.65X_(Cr) is from 15 to 23%, in which X_(V) represents the mass % of the V, X_(Fe) represents the mass % of the Fe, X_(Mo) represents the mass % of the Mo and X_(Cr) represents the mass % of the Cr. Whereby, it is possible to provide a cold workability equivalent to that of Ti-20V-4Al-1Sn. When the said value is less than 15, a titanium alloy comprising only the β-phase is difficult to be obtained even by increasing the cooling rate from the temperature equal to or above the B transformation temperature, and hence the workability is deteriorated due to the precipitation of the martensitic phase, the α-phase or the like. Contrarily to this, when the said value exceeds 23, the precipitation of the α-phase in the aging treatment is inhibited and more excellent strength than Ti-20V-4Al-1Sn is not obtainable.

In order to be able to inhibit the possibility of deteriorating the cold workability due to the precipitation of the α-phase or any phase other than the β-phase, the average cooling rate from the temperature equal to or above the B transformation temperature to at least 500° C., at which a different phase is unlikely to be precipitated, is preferably from 1 to 100° C./sec. Especially, for an intermediate having a value represented by X_(V)+2.95X_(Fe)+1.5X_(Mo)+1.65X_(Cr) being not more than 17%, a different phase is easy to be precipitated due to the low cooling rate, and therefore it is preferably cooled at a rate within the above range. The cooling rate is from 1 to 100° C./sec because when it is 1° C./sec or below, a phase other than the β-phase is easy to be precipitated, and when it is increased to 100° C./sec or above, an effect of preventing the precipitation of a different phase is unlikely to be enhanced.

It is possible to use Nb, Ta, Ni, Mn and Co solely or in combination with each other as β-phase stabilizing elements other than V, Fe, Mo and Cr. In this case, a titanium alloy contains Nb: 0.5 to 2%, Ta: 0.5 to 2%, Ni: 0.25 to 1%, Mn: 0.25 to 1% and Co: 0.25 to 1%, and the value of X_(V)+2.95X_(Fe)+1.5X_(Mo)+165X_(Cr)+0.4X_(Nb)+0.3X_(Ta)+1.6X_(Ni)+2.3X_(Mn)+2.1X_(Co) is from 15 to 23%, in which X_(V) represents the mass % of the V, X_(Fe) represents the mass % of the Fe, X_(Mo) represents the mass % of the Mo, X_(Cr) represents the mass % of the Cr, X_(Nb) represents the mass % of the Nb, X_(Ta) represents the mass % of the Ta, X_(Ni) represents the mass % of the Ni, X_(Mn) represents the mass % of the Mn and X_(Co) represents the mass % of the Co, so that the titanium alloy can have more excellent strength than Ti-20V-4Al-1Sn, while having excellent cold workability. It is possible to use neutral atoms Sn, Zr as optional components solely or in combination by substituting a part of Al therewith according to needs and circumstances. In this case, a titanium alloy contains Sn: not more than 5%, Zr: not more than 5%, and the value of X_(Al)+(X_(Sn)/3)+(X_(Zr)/6) is from 1.5 to 5, in which X_(Al) represents the mass % of the Al, X_(Sn) represents the mass % of the Sn and X_(Zr) represents the mass % of the Zr, so that the titanium alloy has more excellent strength than Ti-20V-4Al-1Sn.

As impurities, inevitable impurities such as O and H exist, and in order to have a good ductility, the content of O is preferably not more than 0.25% by mass, and in order to efficiently improve the strength by an aging treatment, the content of H is preferably not more than 0.05% by mass.

EXAMPLES

Now, the description will be made in more detail for the present invention by citing Examples, without intention to limit the present invention to them.

Examples 1 to 11 Comparative Examples 1 to 6

Each ingot was prepared by button arc melting to have the respective elements contained in each ratio as shown in Table 1, then hot rolled to have a 4 mm thickness plate and then subjected to a solution treatment.

Each intermediate was cooled to 500° C. at an average cooling rate of 4° C./sec after the solution treatment, and then stood to cool at room temperature.

Then, scales were removed, and thus a thin plate specimen of a 1 mm thickness β-type titanium alloy was prepared.

(Evaluation)

The respective Examples and Comparative Examples were evaluated in the manners mentioned below.

<Hot Deformation Resistance>

Hot deformation resistance was determined by the working Formaster test using a test piece (a diameter of 8 mm by a length of 12 mm) cut out from the ingot. Specifically, the test piece was rapidly heated to 900° C. by infra-red radiation, and pressed at a rate of 50 mm/sec with 50% deformation and stress at that time was determined and designated as hot deformation resistance.

<Limit of Cold Rolling Reduction Ratio>

A 4 mm thickness plate prepared by hot rolling of an ingot was subjected to a solution treatment, then cooled, and each surface thereof was cut off by 0.5 mm by machine and scales were removed. Thus, a 3 mm thickness plate was prepared.

Then, end faces were polished by an abrasive paper (#100) and then the plate was cold rolled. The end faces were observed each time a 10% cold rolling is carried out and checked on the presence or absence of cracks.

The reduction ratio, at which one or more cracks each having a 1 mm or more depth from the end faces are caused in every 10 mm, was designated as the limit of cold rolling reduction ratio.

For the limit of cold rolling reduction ratio, an evaluation was made with a value of 70% (a thickness of 0.9 mm) being designated as a maximum value.

<Proof Stress and Tensile Strength>

A 1 mm thickness thin plate specimen was heat treated in vacuum, and a specimen subjected only to a solution treatment (800° C. for 15 min), and a specimen subjected to an aging treatment (500° C. for 8 hrs) following the solution treatment, were prepared. From these heat treated thin plate specimens, half-sized specimens having a parallel portion width of 6.25 mm and a gage length of 25 mm were prepared, and a tensile test was carried out according to JIS Z 2241 at a rate of 0.1 mm/min, and a tensile strength and a 0.2% proof stress were determined. TABLE 1 B-PHASE COMPONENTS (%) STABILIZATION V Fe Cr Mo Al Sn Zr Nb Ta Ni Co Mn Ti INDICES *1 EX. 1 8 1.5 4 0.5 3.5 0 0 0 0 0 0 0 Residue 19.8 EX. 2 8 1 4 2 3.5 0 0 0 0 0 0 0 Residue 20.6 EX. 3 15 1 0.5 0.5 3.5 0 0 0 0 0 0 0 Residue 19.6 EX. 4 8 1.5 4 0.5 3.5 1 0 0 0 0 0 0 Residue 20.5 EX. 5 8 1.5 4 0.5 3.5 0 1 0 0 0 0 0 Residue 20.5 EX. 6 8 1.5 4 2 3.5 0 0 0 0 0 0 0 Residue 22.0 EX. 7 8 1.5 4 0.5 3.5 0 0 0.5 0.5 0 0 0 Residue 20.1 EX. 8 8 1.5 4 0.5 3.5 0 0 0.5 0.5 0.5 0 0 Residue 20.9 EX. 9 8 1.5 4 0.5 3.5 0 0 0.5 0.5 0 0.5 0 Residue 21.3 EX. 10 8 1.5 4 0.5 3.5 0 0 0.5 0.5 0 0 0.5 Residue 21.2 EX. 11 8 1 2 1.5 3.5 0 0 0 0 0 0 0 Residue 16.5 COMP. 8 3 6 0 3.5 0 0 0 0 0 0 0 Residue 26.8 EX. 1 COMP. 8 0.5 0.5 0 3.5 0 0 0 0 0 0 0 Residue 10.5 EX. 2 COMP. 20 0 0 0 3.5 0 0 0 0 0 0 0 Residue 20.0 EX. 3 COMP. 13 0 9 0 3.5 0 0 0 0 0 0 0 Residue 27.9 EX. 4 COMP. 15 0 6 0 4 0 0 0 0 0 0 0 Residue 24.9 EX. 5 COMP. 11 1.5 4 0.5 3.5 0 0 0 0 0 0 0 Residue 23.5 EX. 6 *1: Values represented by Xv + 2.95X_(Fe) + 1.5X_(Mo) + 1.65X_(Cr) + 0.4X_(Nb) + 0.3X_(Ta) + 1.6X_(Ni) + 2.3X_(Mn) + 2.1X_(Co)

TABLE 2 LIMIT OF HOT COLD BEFORE AGING AFTER AGING DEFORMATION ROLLING PROOF TENSILE PROOF TENSILE RESISTANCE REDUCTION STRESS STRENGTH STRESS STRENGTH (MPa) RATIO (%) (MPa) (MPa) (MPa) (MPa) EX. 1 205 70 831 882 1360 1446 EX. 2 190 70 814 857 1316 1394 EX. 3 201 70 654 699 1252 1334 EX. 4 208 70 812 844 1337 1042 EX. 5 207 70 815 852 1345 1412 EX. 6 215 70 820 845 1120 1203 EX. 7 205 70 807 840 1342 1405 EX. 8 210 70 815 845 1331 1395 EX. 9 214 70 810 842 1295 1342 EX. 10 213 70 812 850 1296 1350 EX. 11 193 70 450 725 1210 1305 COMP. EX. 1 233 60 782 839 870 950 COMP. EX. 2 175 20 — — — — COMP. EX. 3 185 70 642 672 1088 1198 COMP. EX. 4 295 70 866 885 940 1009 COMP. EX. 5 270 70 834 856 1034 1146 COMP. EX. 6 225 60 822 860 1050 1190

As Reference Examples 1 to 7, specimens were prepared in the same manner as Example 1, except that an average cooling rate to 500° C. after the solution treatment was set as shown in Table 3, and the result of the observation of the formation of a different phase is shown in Table 3. In the observation, the judgment on the formation of a phase other than the β-phase was made based on a chart by using an X-ray diffraction system. TABLE 3 AVERAGE COOLING RATE TO 500° C. AFTER FORMATION OF A SOLUTION TREATMENT DIFFERENT PHASE REFERENCE EX. 1 50 NOT OBSERVED REFERENCE EX. 2 10 NOT OBSERVED REFERENCE EX. 3 2 NOT OBSERVED REFERENCE EX. 4 80 NOT OBSERVED REFERENCE EX. 5 0.5 α-PHASE WAS OBSERVED REFERENCE EX. 6 0.1 α-PHASE WAS OBSERVED REFERENCE EX. 7 0.01 α-PHASE WAS OBSERVED

As shown above, it is seen that, in Examples 1 to 11, the limit of cold rolling reduction ratio is not lowered as compared with the result of Comparative Example 3 representative of a Ti-20V-4Al-1Sn 8-type titanium alloy, and excellent cold workability equivalent to that of a Ti-20V-4AI-1Sn 8-type titanium alloy is obtained. Also, the proof stress and the tensile strength before and after the aging treatment are high as compared with Comparative Example 3, and thus it is seen that a titanium alloy having more excellent strength than a Ti-20V-4Al-1Sn β-type titanium alloy can be obtained.

Furthermore, it is seen from the results of Reference Examples 1 to 7 that the possibility of precipitating a different phase can be reduced by setting an average cooling rate to 500° C. after the solution treatment within a given range. 

1. A β-type titanium alloy comprising, by mass %, V: 5 to 15%, Fe: 0.5 to 2.5%, Mo: 0.5 to 6% and Cr: 0.5 to 5%, wherein the value of X_(V)+2.95X_(Fe)+1.5X_(Mo)+1.65X_(Cr) is from 15 to 23%, wherein X_(V) represents the mass % of the V, X_(Fe) represents the mass % of the Fe, X_(Mo) represents the mass % of the Mo and X_(Cr) represents the mass % of the Cr, and further comprising, by mass %, Al: 1.5 to 5%, wherein Ti and impurities constitute the residue.
 2. A β-type titanium alloy comprising, by mass %, V: 5 to 15%, Fe: 0.5 to 2.5%, Mo: 0.5 to 6% and Cr: 0.5 to 5%, wherein the value of X_(V)+2.95X_(Fe)+1.5X_(Mo)+1.65X_(Cr) is from 15 to 23%, wherein X_(V) represents the mass % of the V, X_(Fe) represents the mass % of the Fe, X_(Mo) represents the mass % of the Mo and X_(Cr) represents the mass % of the Cr, and further comprising, by mass %, Al: 1.5% to less than 5% and at least one of the group consisting of Sn: not more than 5% and Zr: not more than 5%, wherein the value of X_(Al)+(X_(Sn)/3)+(X_(Zr)/6) is from 1.5 to 5, wherein X_(Al) represents the mass % of the Al, X_(Sn) represents the mass % of the Sn and X_(Zr) represents the mass % of the Zr, wherein Ti and impurities constitute the residue.
 3. A β-type titanium alloy comprising, by mass %, V: 5 to 15%, Fe: 0.5 to 2.5%, Mo: 0.5 to 6%, Cr: 0.5 to 5% and at least one selected from the group consisting of Nb: 0.5 to 2%, Ta: 0.5 to 2%, Ni: 0.25 to 1%, Mn: 0.25 to 1% and Co: 0.25 to 1%, wherein the value of X_(V)+2.95X_(Fe)+1.5X_(Mo)+1.65X_(Cr)+0.4X_(Nb)+0.3X_(Ta)+1.6X_(Ni)+2.3X_(Mn)+2.1X_(Co) is from 15 to 23%, wherein X_(V) represents the mass % of the V, X_(Fe) represents the mass % of the Fe, X_(Mo) represents the mass % of the Mo, X_(Cr) represents the mass % of the Cr, X_(Nb) represents the mass % of the Nb, X_(Ta) represents the mass % of the Ta, X_(Ni) represents the mass % of the Ni, X_(Mn) represents the mass % of the Mn and X_(Co) represents the mass % of the Co, and further comprising, by mass %, Al: 1.5 to 5%, wherein Ti and impurities constitute the residue.
 4. A β-type titanium alloy comprising, by mass %, V: 5 to 15%, Fe: 0.5 to 2.5%, Mo: 0.5 to 6%, Cr: 0.5 to 5% and at least one selected from the group consisting of Nb: 0.5 to 2%, Ta: 0.5 to 2%, Ni: 0.25 to 1%, Mn: 0.25 to 1% and Co: 0.25 to 1%, wherein the value of X_(V)+2.95X_(Fe)+1.5X_(Mo)+1.65X_(Cr)+0.4X_(Nb)+0.3X_(Ta)+1.6X_(Ni)+2.3X_(Mn)+2.1X_(Co) is from 15 to 23%, wherein X_(V) represents the mass % of the V, X_(Fe) represents the mass % of the Fe, X_(Mo) represents the mass % of the Mo, X_(Cr) represents the mass % of the Cr, X_(Nb) represents the mass % of the Nb, X_(Ta) represents the mass % of the Ta, X_(Ni) represents the mass % of the Ni, X_(Mn) represents the mass % of the Mn and X_(Co) represents the mass % of the Co, and further comprising, by mass %, Al: 1.5% to less than 5% and at least one selected from the group consisting of Sn: not more than 5% and Zr: not more than 5%, wherein the value of X_(Al)+(X_(Sn)/3)+(X_(Zr)/6) is from 1.5 to 5, wherein X_(Al) represents the mass % of the Al, X_(Sn) represents the mass % of the Sn and X_(Zr) represents the mass % of the Zr, and wherein Ti and impurities constitute the residue.
 5. A method for heat treatment of the β-type titanium alloy of claim 1, comprising heating a β-type titanium alloy to a temperature equal to or above the β transformation temperature, and then cooling the same at an average cooling rate of 1 to 100° C./sec to at least 500° C. or below.
 6. A method for heat treatment of the β-type titanium alloy of claim 2, comprising heating a β-type titanium alloy to a temperature equal to or above the β transformation temperature, and then cooling the same at an average cooling rate of 1 to 100° C./sec to at least 500° C. or below.
 9. A method for heat treatment of the β-type titanium alloy of claim 3, comprising heating a β-type titanium alloy to a temperature equal to or above the β transformation temperature, and then cooling the same at an average cooling rate of 1 to 100° C./sec to at least 500° C. or below.
 10. A method for heat treatment of the β-type titanium alloy of claim 4, comprising heating a β-type titanium alloy to a temperature equal to or above the β transformation temperature, and then cooling the same at an average cooling rate of 1 to 100° C./sec to at least 500° C. or below. 